There have been proposed a variety of photovoltaic elements for solar cell and for power source in electric appliances. Those photovoltaic elements utilize pn junctions formed by way of ion implantation or thermal diffusion of an impurity into a single crystal substrate such as silicon (Si) or gallium arsenide (GaAs) or by way of epitaxial growth of an impurity-doped layer on such a single crystal substrate. However, there is a disadvantage to any of such known photovoltaic elements since there is used such single crystal substrate as described above, their production cost is inevitably increased and they have not yet come into use generally as solar cell or as power source in electric appliances.
Incidentally, a photovoltaic element utilizing a pin junction comprising an amorphous silicon (hereinafter referred to as "A-Si") film deposited on a substrate of an inexpensive commercially available non-single crystal material such as glass, metal, ceramics and synthetic resin has been proposed in recent years. This photovoltaic element is satisfactory to some extent and it can be prepared at a low cost. In view of this, it has been recognized to be usable as the power source for limited kinds of domestic equipment such as electronic pocket computers and wrist watches. However, for this photovoltaic element, since the band gap in the A-Si film constituting the photovoltatic element is not sufficiently great being of about 1.7 eV, it involves problems that the output voltage is low and no sufficient optoelectronic conversion efficiency can be obtained, this is particularly so, for the light source in which light at short wavelength is predominant, for example, fluorescent lamp and because of this, its application is limited only to appliances with very small power consumption.
There is a further disadvantage for said photovoltaic element that the constituent A-Si film is often accompanied with the so-called Staebler-Wronsk effect, in which the film being deteriorated upon continuous irradiation with intense light over a long period of time.
In view of the above, the photovoltatic element described above can not be put to practical use as the power solar cell requiring to maintain stable characteristics over a long period of time.
Meanwhile, there have been proposed direct transition-type semiconductor films having a wide band gap, such as ZnSe (having a band gap of 2.67 eV) and ZnTe (having a band gap of 2.26 eV) and mixed crystal thereof ZnSe.sub.1-x Te.sub.x (where 0&lt;x&lt;1). Public attention has been focused on these semiconductor films. These semiconductor films are, in general, such that are formed on a substrate of single crystal by way of epitaxial growth. The as-grown film of ZnSe exhibits the n-type conductivity and the as-grown film of ZnTe exhibits the p-type conductivity. However for any of these films, it is generally recognized that it is difficult for the film to be controlled to the opposite conduction type. Further, in order to carry out the epitaxial growth upon the film formation, it is required to use a specific substrate of single crystal and to maintain the substrate at elevated temperature. And in this film formation, the deposition rate is low. Because of this, it is impossible to perform epitaxial growth on a commercially available substrate which is inexpensive and low heat-resistant such as glass and synthetic resin. These factors make it difficult to develop practically applicable semiconductors films using the foregoing commercially available substrates.
Even in the case where a semiconductor film should be fortunately formed on such commercially available substrate, the film will be such that is usable only in very limited applications.
In fact, there have been various proposals to form a direct transition-type semiconductor film on a non-single crystal substrate such as glass, metal, ceramics and synthetic resin. However, under any of such proposals, it is difficult to obtain a desired direct transition-type semiconductor film having satisfactory electrical characteristics because the resulting film is accompanied with defects of various kinds which make the film poor in electrical characteristics and because of this, it is difficult for the film to be controlled by doping it with an impurity.
By the way, proposals for amorphous films constituted with Zn, Se, Te elements can be seen in the literature. As such literature, there can be mentioned U.S. Pat. No. 4217374 (hereinafter referred to as "literature 1") and U.S. Pat. No. 4226898 (hereinafter referred to as "literature 2"). Furthermore, ZnSe compound or ZnTe compound is described in Japanese Patent Laid-Open No. Sho 61-189649 (hereinafter referred to as "literature 3") and Japanese Patent Laid-Open No. Sho 61-189650 (hereinafter referred to as "literature 4"). Although the literature 1 mentions an amorphous semiconductor film containing selenium (Se), tellurium (Te), zinc (Zn), hydrogen (H) and lithium (Li), it mainly describes amorphous Se semiconductor film or amorphous Te semiconductor film, in which Zn is stated only as an additive similar to Li and H. As for the Zn and the Li, they are incorporated in order to reduce the localized level density in the energy gap without changing the inherent nature of the film similar to the case of the H. In other words, the additions of Zn and Se in the literature 1 are not intended to positively form a ZnSe compound. In fact, the literature 1 mentions nothing at all about the formation of a ZnSe compound, ZnTe compound or ZnSe.sub.1-x Te.sub.x compound and likewise, nothing at all about the formation of ZnSe crystal grains, ZnTe crystal grains and ZnSe.sub.1-x Te.sub.x crystal grains. In addition, Li is not added as the dopant.
Although the literature 2 describes an amorphous semiconductor film containing Se, Te, Zn and H, the description in the literature 2 mainly concerns amorphous Si, and the Se and the Te are described as forming compounds therewith, while the Zn is described as the element for sensitizing the photoconductivity and reducing the localized level density in the energy gap of the amorphous semiconductor film. That is, Zn, Se and Te are added not for forming a ZnSe compound, ZnTe compound or ZnSe.sub.1-x Te.sub.x compound. The literature 2 mentions nothing at all about the formation of a ZnSe compound, ZnTe compound or ZnSe.sub.1-x Te.sub.x compound and likewise, nothing at all about the formation of ZnSe crystal grains, ZnTe crystal grains and ZnSe.sub.1-x Te.sub.x grains.
Literatures 3 and 4 are of the contents of improving the productivity of ZnSe and ZnTe films upon forming them by deposition through the hydrogen radical assisted CVD method (HR-CVD method) by increasing the film depositing rate, but they merely disclose the deposited films constituted with non-doped ZnSe and ZnTe.
Against these backgrounds, there is an increased social demand to provide an inexpensive photovoltaic element having a high photoelectric conversion efficiency, particularly, for short-wavelength light which may be practically usable as solar cell and also as a power source in various electric appliances.